Cassini Team Delivers New Findings on Saturn’s Ring System

Although NASA’s Cassini mission ended in 2017, science continues to flow from the data collected. In a series of papers in the journal Science, planetary researchers analyzed data from Cassini’s ring grazing orbits (December 2016 to April 2017) and its Grand Finale phase (April to September 2017). Key among the findings is the discovery that Saturn’s rings are relatively young, probably just 10-100 million years old.

NASA’s Cassini spacecraft in orbit around Saturn. Image credit: NASA / JPL-Caltech.

NASA’s Cassini spacecraft in orbit around Saturn. Image credit: NASA / JPL-Caltech.

“Saturn’s rings are an accessible exemplar of astrophysical disk processes and a delicate tracer of the Saturn system’s dynamical processes and history,” said Cassini scientist Matt Tiscareno of the SETI Institute and colleagues.

“During its ring grazing orbits and Grand Finale, Cassini passed very close to Saturn’s main rings and obtained very high-spatial-resolution images, spectral scans, and temperature scans.”

Dr. Tiscareno’s team analyzed those images and scans and discovered complex features sculpted by the gravitational interactions between Saturnian moons and ring particles.

The false-color image at right shows spectral mapping of Saturn’s A, B and C rings, captured by Cassini’s Visible and Infrared Mapping Spectrometer (VIMS). It displays an infrared view of the rings, rather than an image in visible light. The blue-green areas are the regions with the purest water ice and/or largest grain size (primarily the A and B rings), while the reddish color indicates increasing amounts of non-icy material and/or smaller grain sizes (primarily in the C ring and Cassini Division). At left, the same image is overlaid on a natural-color mosaic of Saturn taken by Cassini’s Imaging Science Subsystem (ISS). Image credit: NASA / JPL-Caltech / Space Science Institute / G. Ugarkovic / University of Arizona / CNRS / LPG-Nantes.

The false-color image at right shows spectral mapping of Saturn’s A, B and C rings, captured by Cassini’s Visible and Infrared Mapping Spectrometer (VIMS). It displays an infrared view of the rings, rather than an image in visible light. The blue-green areas are the regions with the purest water ice and/or largest grain size (primarily the A and B rings), while the reddish color indicates increasing amounts of non-icy material and/or smaller grain sizes (primarily in the C ring and Cassini Division). At left, the same image is overlaid on a natural-color mosaic of Saturn taken by Cassini’s Imaging Science Subsystem (ISS). Image credit: NASA / JPL-Caltech / Space Science Institute / G. Ugarkovic / University of Arizona / CNRS / LPG-Nantes.

“At the outer edge of the main rings, a series of similar impact-generated streaks in the F ring have the same length and orientation, showing that they were likely caused by a flock of impactors that all struck the ring at the same time,” the scientists said.

This shows that the ring is shaped by streams of material that orbit Saturn itself rather than, for instance, by cometary debris (moving around the Sun) that happens to crash into the rings.

“These new details of how the moons are sculpting the rings in various ways provide a window into the formation of the Solar System, where you also have disks evolving under the influence of masses embedded within them,” Dr. Tiscareno said.

This enhanced-color image mosaic shows Daphnis in the Keeler gap on the sunlit side of the rings. Daphnis is seen kicking up three waves in the gap’s outer edge. Image credit: NASA / JPL-Caltech / Space Science Institute.

This enhanced-color image mosaic shows Daphnis in the Keeler gap on the sunlit side of the rings. Daphnis is seen kicking up three waves in the gap’s outer edge. Image credit: NASA / JPL-Caltech / Space Science Institute.

A team led by Dr. Bonnie Buratti from NASA’s Jet Propulsion Laboratory analyzed Cassini observations of the five small moons located in and around Saturn’s rings.

“Saturn’s main ring system is associated with a family of small moons,” the researchers said.

“Pan and Daphnis orbit within the A-ring’s Encke Gap and Keeler Gap, respectively, whereas Pandora and Prometheus orbit just outside the F-ring and Atlas just outside the A-ring.”

“The moons Janus and Epimetheus are in closely spaced orbits that they exchange approximately every 4 years; these two objects may be collisional fragments of a larger body.”

“Five close flybys of the moons Pan, Daphnis, Atlas, Pandora, and Epimetheus were performed between December 2016 and April 2017 during the ring-grazing orbits of the Cassini mission.”

“Data on the moons’ morphology, structure, particle environment, and composition were returned, along with images in the ultraviolet and thermal infrared.”

During super-close flybys of Saturn’s rings, Cassini inspected the mini-moons Pan and Daphnis in the A ring; Atlas at the edge of the A ring; Pandora at the edge of the F ring; and Epimetheus, which is bathed in material that fans out from the moon Enceladus. Image credit: NASA-JPL / Caltech.

During super-close flybys of Saturn’s rings, Cassini inspected the mini-moons Pan and Daphnis in the A ring; Atlas at the edge of the A ring; Pandora at the edge of the F ring; and Epimetheus, which is bathed in material that fans out from the moon Enceladus. Image credit: NASA-JPL / Caltech.

They found that the optical properties of the moons’ surfaces are determined by two competing processes: contamination by a red material formed in Saturn’s ring system and accumulation of bright icy particles or water vapor from volcanic plumes originating on the moon Enceladus.

“We found these moons are scooping up particles of ice and dust from the rings to form the little skirts around their equators,” Dr. Buratti said.

“A denser body would be more ball-shaped because gravity would pull the material in.”

During the Grand Finale, NASA’s Cassini spacecraft passed between the inner edge of Saturn’s D-ring and the cloud top. This orbital configuration allowed the disentanglement of the tiny acceleration of the rings from the large acceleration due to Saturn. The two forces pull the spacecraft in opposite directions. Image credit: NASA / JPL-Caltech / Iess et al.

During the Grand Finale, NASA’s Cassini spacecraft passed between the inner edge of Saturn’s D-ring and the cloud top. This orbital configuration allowed the disentanglement of the tiny acceleration of the rings from the large acceleration due to Saturn. The two forces pull the spacecraft in opposite directions. Image credit: NASA / JPL-Caltech / Iess et al.

A research team led by Dr. Luciano Iess from the Sapienza University of Rome measured the gravitational pull on Cassini, separating the contributions from the planet and the rings.

This allowed them to determine the interior structure of Saturn and the mass of its rings.

“Saturn’s gravity field measured by Cassini implies a strong and deep differential rotation, extending to a depth of about 5,600 miles (9,000 km). This differs from Jupiter, where winds are shallower (1,864 miles, or about 3,000 km),” they said.

“The gravity measurements are consistent with a mass of Saturn’s core of 15 to 18 Earth masses.”

The scientists found that the total mass of the rings is 1.54*1019 — about 0.41 times that of the Saturnian moon Mimas.

“The low value of the ring mass suggests a scenario where the present rings of Saturn are young, probably just 10 million to 100 million years old, to be consistent with their pristine icy composition,” they said.

“Models for a young ring system invoke the chance capture and tidal disruption of a comet or an icy body from the outer Solar System, suggesting that catastrophic events continued to occur in the Solar System long after its formation 4.6 billion years ago.”

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Matthew S. Tiscareno et al. 2019. Close-range remote sensing of Saturn’s rings during Cassini’s ring-grazing orbits and Grand Finale. Science 364 (6445): eaau1017; doi: 10.1126/science.aau1017

B.J. Buratti et al. 2019. Close Cassini flybys of Saturn’s ring moons Pan, Daphnis, Atlas, Pandora, and Epimetheus. Science 364 (6445): eaat2349; doi: 10.1126/science.aat2349

L. Iess et al. 2019. Measurement and implications of Saturn’s gravity field and ring mass. Science 364 (6445): eaat2965; doi: 10.1126/science.aat2965

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